I think Mick J's points are interesting.
The fuel pumps were replaced. Assuming this was the result of intelligent diagnosis, you may indeed have a faulty fuel pump relay. When fuel pumps get old, they often times produce excess amperage which can damage/destroy a fuel pump relay.
Jumpering the pins as Mick suggested could answer some questions.
There have been many write ups here regarding this process and there may even be some visual info as well in a previous post(s). Try the SEARCH facility for help in this area.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.